AOT metadata errors link
The following are metadata errors you may encounter, with explanations and suggested corrections.
Expression form not supported
Reference to a local (non-exported) symbol
Only initialized variables and constants
Reference to a non-exported class
Reference to a non-exported function
Function calls are not supported
Destructured variable or constant not supported
Could not resolve type
Name expected
Unsupported enum member name
Tagged template expressions are not supported
Symbol reference expected
Expression form not supported link
The compiler encountered an expression it didn't understand while evaluating Angular metadata.
Language features outside of the compiler's restricted expression syntax can produce this error, as seen in the following example:
// ERROR
export class Fooish { … }
…
const prop = typeof Fooish; // typeof is not valid in metadata
…
// bracket notation is not valid in metadata
{ provide: 'token', useValue: { [prop]: 'value' } };
…
You can use
typeof
and bracket notation in normal application code.
You just can't use those features within expressions that define Angular metadata.
Avoid this error by sticking to the compiler's restricted expression syntax when writing Angular metadata and be wary of new or unusual TypeScript features.
Reference to a local (non-exported) symbol link
Reference to a local (non-exported) symbol 'symbol name'. Consider exporting the symbol.
The compiler encountered a referenced to a locally defined symbol that either wasn't exported or wasn't initialized.
Here's a
provider
example of the problem.
// ERROR
let foo: number; // neither exported nor initialized
@Component({
selector: 'my-component',
template: … ,
providers: [
{ provide: Foo, useValue: foo }
]
})
export class MyComponent {}
The compiler generates the component factory, which includes the
useValue
provider code, in a separate module.
That
factory module can't reach back to
this
source module to access the local (non-exported)
foo
variable.
You could fix the problem by initializing
foo
.
let foo = 42; // initialized
The compiler will fold the expression into the provider as if you had written this.
providers: [
{ provide: Foo, useValue: 42 }
]
Alternatively, you can fix it by exporting
foo
with the expectation that
foo
will be assigned at runtime when you actually know its value.
// CORRECTED
export let foo: number; // exported
@Component({
selector: 'my-component',
template: … ,
providers: [
{ provide: Foo, useValue: foo }
]
})
export class MyComponent {}
Adding
export
often works for variables referenced in metadata such as
providers
and
animations
because the compiler can generate
references
to the exported variables in these expressions. It doesn't need the
values
of those variables.
Adding
export
doesn't work when the compiler needs the
actual value
in order to generate code.
For example, it doesn't work for the
template
property.
// ERROR
export let someTemplate: string; // exported but not initialized
@Component({
selector: 'my-component',
template: someTemplate
})
export class MyComponent {}
The compiler needs the value of the
template
property
right now
to generate the component factory.
The variable reference alone is insufficient.
Prefixing the declaration with
export
merely produces a new error, "
Only initialized variables and constants can be referenced
".
Only initialized variables and constants link
Only initialized variables and constants can be referenced because the value of this variable is needed by the template compiler.
The compiler found a reference to an exported variable or static field that wasn't initialized. It needs the value of that variable to generate code.
The following example tries to set the component's
template
property to the value of the exported
someTemplate
variable which is declared but
unassigned
.
// ERROR
export let someTemplate: string;
@Component({
selector: 'my-component',
template: someTemplate
})
export class MyComponent {}
You'd also get this error if you imported
someTemplate
from some other module and neglected to initialize it there.
// ERROR - not initialized there either
import { someTemplate } from './config';
@Component({
selector: 'my-component',
template: someTemplate
})
export class MyComponent {}
The compiler cannot wait until runtime to get the template information.
It must statically derive the value of the
someTemplate
variable from the source code so that it can generate the component factory, which includes instructions for building the element based on the template.
To correct this error, provide the initial value of the variable in an initializer clause on the same line .
// CORRECTED
export let someTemplate = '<h1>Greetings from Angular</h1>';
@Component({
selector: 'my-component',
template: someTemplate
})
export class MyComponent {}
Reference to a non-exported class link
Reference to a non-exported class
<class name>
.
Consider exporting the class.
Metadata referenced a class that wasn't exported.
For example, you may have defined a class and used it as an injection token in a providers array but neglected to export that class.
// ERROR
abstract class MyStrategy { }
…
providers: [
{ provide: MyStrategy, useValue: … }
]
…
Angular generates a class factory in a separate module and that factory can only access exported classes. To correct this error, export the referenced class.
// CORRECTED
export abstract class MyStrategy { }
…
providers: [
{ provide: MyStrategy, useValue: … }
]
…
Reference to a non-exported function link
Metadata referenced a function that wasn't exported.
For example, you may have set a providers
useFactory
property to a locally defined function that you neglected to export.
// ERROR
function myStrategy() { … }
…
providers: [
{ provide: MyStrategy, useFactory: myStrategy }
]
…
Angular generates a class factory in a separate module and that factory can only access exported functions. To correct this error, export the function.
// CORRECTED
export function myStrategy() { … }
…
providers: [
{ provide: MyStrategy, useFactory: myStrategy }
]
…
Function calls are not supported link
Function calls are not supported. Consider replacing the function or lambda with a reference to an exported function.
The compiler does not currently support function expressions or lambda functions.
For example, you cannot set a provider's
useFactory
to an anonymous function or arrow function like this.
// ERROR
…
providers: [
{ provide: MyStrategy, useFactory: function() { … } },
{ provide: OtherStrategy, useFactory: () => { … } }
]
…
You also get this error if you call a function or method in a provider's
useValue
.
// ERROR
import { calculateValue } from './utilities';
…
providers: [
{ provide: SomeValue, useValue: calculateValue() }
]
…
To correct this error, export a function from the module and refer to the function in a
useFactory
provider instead.
// CORRECTED
import { calculateValue } from './utilities';
export function myStrategy() { … }
export function otherStrategy() { … }
export function someValueFactory() {
return calculateValue();
}
…
providers: [
{ provide: MyStrategy, useFactory: myStrategy },
{ provide: OtherStrategy, useFactory: otherStrategy },
{ provide: SomeValue, useFactory: someValueFactory }
]
…
Destructured variable or constant not supported link
Referencing an exported destructured variable or constant is not supported by the template compiler. Consider simplifying this to avoid destructuring.
The compiler does not support references to variables assigned by destructuring.
For example, you cannot write something like this:
// ERROR
import { configuration } from './configuration';
// destructured assignment to foo and bar
const {foo, bar} = configuration;
…
providers: [
{provide: Foo, useValue: foo},
{provide: Bar, useValue: bar},
]
…
To correct this error, refer to non-destructured values.
// CORRECTED
import { configuration } from './configuration';
…
providers: [
{provide: Foo, useValue: configuration.foo},
{provide: Bar, useValue: configuration.bar},
]
…
Could not resolve type link
The compiler encountered a type and can't determine which module exports that type.
This can happen if you refer to an ambient type.
For example, the
Window
type is an ambient type declared in the global
.d.ts
file.
You'll get an error if you reference it in the component constructor, which the compiler must statically analyze.
// ERROR
@Component({ })
export class MyComponent {
constructor (private win: Window) { … }
}
TypeScript understands ambient types so you don't import them. The Angular compiler does not understand a type that you neglect to export or import.
In this case, the compiler doesn't understand how to inject something with the
Window
token.
Do not refer to ambient types in metadata expressions.
If you must inject an instance of an ambient type, you can finesse the problem in four steps:
- Create an injection token for an instance of the ambient type.
- Create a factory function that returns that instance.
-
Add a
useFactoryprovider with that factory function. -
Use
@Injectto inject the instance.
Here's an illustrative example.
// CORRECTED
import { Inject } from '@angular/core';
export const WINDOW = new InjectionToken('Window');
export function _window() { return window; }
@Component({
…
providers: [
{ provide: WINDOW, useFactory: _window }
]
})
export class MyComponent {
constructor (@Inject(WINDOW) private win: Window) { … }
}
The
Window
type in the constructor is no longer a problem for the compiler because it
uses the
@Inject(WINDOW)
to generate the injection code.
Angular does something similar with the
DOCUMENT
token so you can inject the browser's
document
object (or an abstraction of it, depending upon the platform in which the application runs).
import { Inject } from '@angular/core';
import { DOCUMENT } from '@angular/common';
@Component({ … })
export class MyComponent {
constructor (@Inject(DOCUMENT) private doc: Document) { … }
}
Name expected link
The compiler expected a name in an expression it was evaluating.
This can happen if you use a number as a property name as in the following example.
// ERROR
provider: [{ provide: Foo, useValue: { 0: 'test' } }]
Change the name of the property to something non-numeric.
// CORRECTED
provider: [{ provide: Foo, useValue: { '0': 'test' } }]
Unsupported enum member name link
Angular couldn't determine the value of the enum member that you referenced in metadata.
The compiler can understand simple enum values but not complex values such as those derived from computed properties.
// ERROR
enum Colors {
Red = 1,
White,
Blue = "Blue".length // computed
}
…
providers: [
{ provide: BaseColor, useValue: Colors.White } // ok
{ provide: DangerColor, useValue: Colors.Red } // ok
{ provide: StrongColor, useValue: Colors.Blue } // bad
]
…
Avoid referring to enums with complicated initializers or computed properties.
Tagged template expressions are not supported link
Tagged template expressions are not supported in metadata.
The compiler encountered a JavaScript ES2015 tagged template expression such as the following.
// ERROR
const expression = 'funky';
const raw = String.raw`A tagged template ${expression} string`;
…
template: '<div>' + raw + '</div>'
…
String.raw()
is a
tag function
native to JavaScript ES2015.
The AOT compiler does not support tagged template expressions; avoid them in metadata expressions.
Symbol reference expected link
The compiler expected a reference to a symbol at the location specified in the error message.
This error can occur if you use an expression in the
extends
clause of a class.
